Leakage of petroleum products, such as gasoline, into the environment may be damaging to surrounding soil and water. Because the amount of environmental damage caused by the leak depends in part on the total amount of product lost, the environmental clean up or remediation will tend to be more costly and time consuming the longer a leak is allowed to continue. Thus, it is desirable to identify leaks as early as possible.
In a dispensing operation, such as at a gas station, fuel is typically stored in underground storage tanks (“UST”) from where it is pumped through various conduit lines to one or more above ground dispensing units for dispensing into motor vehicles or the like. Fuel leaks from the UST or the interconnecting conduit lines can cause significant environmental damage. The United States Environmental Protection Agency (“EPA”), as well as regulatory bodies in many other countries, have set certain standards for the detection and prevention of environmental leaks of fuel. For example, as this application is filed, the EPA requires detection methods sufficient to detect volumetric line leak rates of as little as 0.1 gallons per hour (“GPH”). Accordingly, fuel dispensing equipment manufacturers have been developing methods and systems to detect leaks with sufficient accuracy to meet EPA standards in this regard.
A number of devices operating on a variety of physical principles have been proposed to meet these standards, thereby warning of leaks and providing means for stopping the leaks as quickly as possible to reduce the impact on the surrounding environment. By way of example, in fuel dispensing operations, one such type of leak detector device includes a valve disposed in the conduit line to isolate a downstream section of the conduit line from the UST when fuel is not being dispensed. The leak detector valve includes a valve element held in place against an associated valve seat by a spring or other device so that the valve is normally in a closed position. When the pressure drop across the valve element (e.g., in a forward direction) reaches a threshold, commonly referred to as the cracking pressure, the valve element is forced open by the positive pressure across the valve, allowing fluid to flow through the valve and toward the dispensing unit so that fuel may be dispensed to a motor vehicle or the like. When the dispensing unit shut off valve or nozzle is closed, or the dispensing operation is otherwise terminated, the forward pressure drop across the valve element abates, and the valve returns to a closed state so as to prevent any fuel from passing through the valve.
The leak detection function of these devices may be provided by a bypass line around the primary valve such that one end thereof is in fluid communication with a first side of the valve (e.g., downstream side) and the other end of the bypass line is in fluid communication with a second side of the valve (e.g., upstream side). A flow detector is typically disposed in the bypass line so as to detect any flow therethrough. To allow relatively small leaks in the conduit line to be detected, the bypass line typically has a relatively small cross-sectional area relative to the primary flow path through the valve.
Conduit line leak detection systems include pressure decay systems and volumetric based systems. In a pressure decay system, the line is pressurized by turning on a submersible turbine pump (“STP”), located in the UST for example, until the conduit line is fully pressurized. The STP is then shut off, with a check valve between the STP and the leak detector maintaining pressure on the upstream side of the leak detector by preventing fuel from flowing back into the UST. Ideally, there is no leak in the conduit line and thus no flow through the bypass line. However, if there is a leak in the conduit line downstream of the leak detector valve, the pressure in the downstream conduit line will steadily decrease. This pressure drop will, in turn, cause fluid to flow from the upstream side of the valve (i.e., the high pressure side) to the downstream side of the valve (i.e., the low pressure side) through the bypass line. The flow detector will then detect this flow through the bypass line and cause an alarm condition, which may shut down the dispensing system to prevent any further leakage of fuel from the conduit line to the surrounding environment.
Volume based systems work on a similar principle as described above for a pressure decay system. However, volume based systems keep the STP running during the leak test to maintain constant pressure on the upstream side of the leak detector. During the leak test, the system monitors the flow of fluid through the bypass line. If there are no leaks in the conduit line, the flow rate will drop during the test as the pressure equalizes on both sides of the leak detector, with the flow rate ultimately reaching zero. However, if there is a leak in the conduit line, the flow rate will settle on a steady non-zero value equal to the rate fluid is being lost from the conduit line.
Both pressure decay and volume based systems are conventionally designed to prevent reverse fluid flow within the bypass line, either by the presence of a check valve within the conduit line or by maintaining positive pressure within the line during leak detection. The leak detector has therefore traditionally been used to detect fluid flow only in the forward direction, and known methods of leak detection rely on these positive flow measurements.
Leak detection devices operating on the basic principles outlined above are generally known in the art. By way of example, U.S. Pat. No. 3,940,020 to McCrory et al.; U.S. Pat. No. 3,969,923 to Howell; U.S. Pat. No. 5,014,543 to Franklin et al.; U.S. Pat. Nos. 5,072,621 and 5,315,862 to Hasselmann; and U.S. Pat. No. 5,918,268 to Lukas et al. generally show a valve, a bypass line, and some type of flow detector for detecting flow through the bypass line. These references differ primarily in the flow detector used to detect flow through the bypass line. For example, McCrory et al. and Howell use a reed switch in conjunction with a magnetized piston to sense flow through the bypass line. Franklin et al. use a rotometer to measure the fluid flow through a bypass line. Moreover, Lukas et al. utilize a thermal flow meter that operates on generally well known principles for determining the flow through the bypass line.
Another example of a leak detector using a bypass line is described in U.S. Patent Pub. No. 20100281953, entitled “Line Leak Detector and Method of Using Same”, the disclosure of which is incorporated herein by reference in its entirety, and assigned to the assignee of the present application.
While the methods of leak detection described above generally work for their intended purpose, there are some drawbacks that make accurate detection of conduit line leaks problematic in a typical fuel dispensing operation. For example, the fuel in the UST is often at a different temperature than the ground surrounding the conduit line, so that immediately after a fuel dispensing operation, the fuel in the conduit line will not be in thermal equilibrium with its surrounding environment. In cases where the fuel is at a higher temperature than the ground surrounding the conduit line, the volume of the fuel will contract as its temperature drops over time, lowering the pressure in the conduit line. This drop in pressure may cause fuel to flow through the bypass line, potentially triggering a false leak alarm. Conversely, if the fuel in the UST is at a lower temperature than the ground surrounding the conduit line, the volume of the fuel in the line will undergo thermal expansion as the fuel warms. This volumetric expansion may potentially conceal or reduce the apparent magnitude of a leak by reducing the flow through the bypass line required to replace the fuel lost due to the leak. Thus, a precision line leak test typically cannot be performed with sufficient accuracy to detect a 0.1 GPH leak until the fuel in the conduit line is closer in temperature to the surrounding environment. When the fuel's temperature is close enough to that of the environment that it no longer disrupts the 0.1 GPH line leak test, say that the fuel has reached “thermal stability”.
Because the temperature of the fuel in the conduit line is typically disturbed whenever a fuel dispensing event occurs, operators may not be able to conduct accurate conduit line leak tests for a significant period of time after a customer purchases fuel. The amount of time required for the fuel to reach thermal stability will vary depending on the initial temperature difference, the thermal mass of the fuel contained in the conduit line, and possibly other factors. In any event, it may be as long as several hours for the fuel to reach stability. This uncertainty about the thermal relationship between the fuel in the conduit line and its surrounding environment may result in having to wait an artificially long time before conducting a line leak test in order to ensure that the fuel temperature has sufficiently stabilized. Waiting significantly longer than required by the actual temperature differences between the fuel and its surrounding environment to conduct a line leak test poses a significant problem for fuel dispensing operations that operate continuously, such as gas stations along busy interstate highways, since fuel may not be dispensed during the waiting period.
Additionally, a line leak test may typically take as much as twelve hours to perform, during which fuel cannot be dispensed. As this represent a significant opportunity cost for a busy gas station, it is critical that the test only be initiated when it will yield accurate results.
Consequently, there is a need for improved methods of line leak detection that cannot only detect small leaks in a fluid conduit line so as to meet or exceed EPA standards, but that can also determine when the fuel in the conduit line has sufficient thermal stability to allow accurate leak detection to be conducted.